Rheumatoid arthritis is a prevalent disease characterized by chronic inflammation of the synovial membrane lining the afflicted joint. Current treatment methods for severe cases of rheumatoid arthritis include the removal of the synovial membrane, e.g., synovectomy. Surgical synovectomy has many limitations including the risk of the surgical procedure itself, and the fact that a surgeon often cannot remove all of the membrane. The diseased tissue remaining may eventually regenerate, causing the same symptoms which the surgery was meant to alleviate.
Radiation synovectomy is radiation-induced ablation of diseased synovial membrane tissue accomplished by injecting a radioactive compound into the diseased synovium. Early attempts to perform radiation synovectomy were hampered by an instability of the radioactive compositions utilized and by leakage of such compounds from the synovium into surrounding healthy tissues. The instability of labile radionuclide-complexes resulted in displacement of the radionuclide from the colloid complex and retention of the radionuclide in soft tissues. Significant leakage of the radioactive compound from the injection site exposed normal tissues to dangerous levels of radiation. Because of these limitations, new radiolabeled compositions were sought which would have minimal leakage.
U.S. Pat. Nos. 4,752,464; 4,849,209 and 3,906,450 describe compositions comprising a radioactive colloid in which a radionuclide is entrapped within an iron hydroxide matrix. The radioactive colloids are useful in radiation ablation procedures, for example, ablation of a synovium in rheumatoid arthritis. However, the use of radioactive colloids may still result in significant leakage of radioactivity from a site of injection, e.g., a synovium, and into the surrounding normal tissues, exposing normal tissues to an undesirable amount of radiation. To compensate for the leakage, a radioactive metal having a short half-life, such as .sup.165 dysprosium (Dy-165) with a half-life of 2.3 hours, has been proposed for use as the therapeutic radionuclide. Because of its short half-life, the majority of Dy-165 radioactivity decays before significant leakage can occur, thereby minimizing the dose of radiation to normal tissues.
The use of radioactive metals having a short half-life severely limits the utility of the therapeutic radiation procedure in two significant ways. First, radioactive compositions prepared with short half-life isotopes lose a significant amount of radioactivity because of decay during shipment to distant locations. Second, to achieve a therapeutic dose of a composition comprising a radioactive metal having a short half-life, large amounts of radioactive materials must be used. As a result, clinical personnel must handle large amounts of radioactive materials.
Therefore, there remains a need for a therapeutic radioactive composition which upon injection into a synovium, would remain at the site of injection, e.g., within a synovium, for a prolonged period of time. Prolonged retention at the site of injection would allow use of radionuclides having a longer half-life in therapeutic procedures, including radiation synovectomy, without fear of significant leakage from the site of injection and radiation exposure to normal tissues.